I've never had full out sex in front of people, only some heavy petting. One gf and I went to the side of a house during a party (others could see) for a bit of action.
I have done it in a high school stairwell on many occasions with that gf. That was very fun, we had to keep our clothes mostly on and listen for people coming.
I think it would be fun to try to do it in front of others without them knowing. I don't think my wife would go for it, she's a bit conservative.
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